The current global concern for the environment has generated a strong need by both government and industry for technologies that reduce emissions of NO.sub.x. NO.sub.x are primary contributors to photochemical smog and acid rain, and may deplete the ozone layer. Up to millions of tons of nitrogen oxides, generally denoted herein as "NO.sub.x," are emitted into the atmosphere each year as a result of numerous industrial and military processes, ranging from high temperature combustion of fossil fuels, to explosive manufacturing and munitions disposal processing and further to operations of powered aerospace ground equipment.
The impact of such emissions on human health and the environment in general has been the subject of intense study and public debate and legislative action to mandate safer emissions has already been enacted. For example, the Clean Air Act Amendment of 1990 mandates that emission generating industrial plants develop and/or implement techniques to significantly reduce their emissions of NO.sub.x. Such legislation affects power plants, iron and steel plants, pulp and paper mills, acid production plants, petroleum refineries, lime plants, fuel conversion plants, glass fiber processing plants, charcoal production plants, cement plants, copper smelters, coal cleaning plants, etc.
Developmental efforts have been directed to converting NO.sub.x to the individual elemental diatoms, N.sub.2 and O.sub.2. Conventional processes typically utilized thermal techniques for generating very high temperature conditions within a reactor. These techniques are highly inefficient as excessively high electrical power is needed not only to treat relatively low pollutant concentrations, but to cool the resultant effluent emerging from the reactor.
Electron beam irradiation has also been used in various forms to convert to the individual elemental diatoms. Such systems ALSO use electron beams or ultraviolet light to oxidize the NO.sub.x. The ionization caused by the electron beam irradiation converts the NO.sub.x to acid mist at low temperatures and/or solid particles at high temperatures which may be removed by conventional methods employing filters and scrubbers. However, due to potential harm to operation personnel, costly and elaborate shielding measures must be employed.
Numerous research agencies have investigated the use of non-thermal plasma devices ("NTPDs") in processes that reduce NOX in gas streams. These techniques use exogenous reducing agents, such as ammonia (NH.sub.3), methane (CH.sub.4), or carbon monoxide (CO), or neutralizing agents, such as calcium hydroxide (Ca (OH).sub.2). The techniques have utility; however, they are accomplished with relatively low efficiency levels.
Accordingly, there exists a demand for non-thermal plasma method and apparatus for treating NO.sub.x bearing gas streams with significantly improved efficiency. It is also desired that such method and apparatus have significantly increased applicability and ease of application. In accordance with the present invention, a non-thermal plasma method and apparatus are employed, utilizing ethanol (C.sub.2 H.sub.5 OH) as a preinjectant in improving the efficiency of NO.sub.x removal from effluent or exhaust gas. In certain instances, the efficiency (in terms of energy per molecule of remediated NO.sub.x) and total reduction (in terms of percentage of hazardous compounds reduced per treatment pass) may be increased up to a factor of ten when compared with conventional non-thermal plasma method and devices.
The non-thermal plasma treatment of the instant invention involves the pretreatment of the exhaust gas with ethanol, either by vapor absorption or direct vapor injection. In particular, before the exhaust gas is exposed to the plasma chamber, at least a portion of the exhaust gas is exposed to an ethanol bath or an ethanol vapor injector such that ethanol vapor is absorbed into the exhaust gas. Advantageously, the high vapor pressure of ethanol permits a significant portion of the ethanol to be absorbed by the exhaust gas. As the ethanol-bearing exhaust gas is further exposed to the reactive species generated by the plasma reactor, e.g., oxygen atoms, nitric oxides are converted to a variety of products, including primarily nitrogen dioxide, with significantly improved efficiency. Moreover, the solubility of ethanol permits the ethanol to be readily scrubbed from the exhaust gas downstream of the plasma reactor, along with the converted nitrogen dioxide.
To implement these treatment methods, a non-thermal plasma apparatus is provided, comprising a non-thermal plasma reactor having a reaction or plasma chamber which provides the reaction zone defined by a plurality of electrodes and dielectrics. The apparatus further comprises an inlet and an outlet connected to the reaction chamber, permitting the exhaust gas to enter and leave the reaction chamber. In one embodiment, the inlet is further connected to a diverter equipped with an injector, which diverts a portion of the gas stream through an ethanol bath before reinjecting the ethanol-bearing gas stream into the inlet. Ethanol is readily absorbed by the gas stream as it passes through the ethanol bath.
In another embodiment, the inlet is equipped with an injector which receives a supply of ethanol that is sprayed as a fine mist directly into the gas stream. The fine mist of ethanol is substantially uniformly absorbed by the gas stream before it enters the reaction chamber. A reservoir stores the ethanol which is delivered to the injector by a metered pump.
The non-thermal plasma apparatus of the present invention may employ either embodiment depending on the concentration of ethanol to be used and/or the flow rate and temperature of the gas. Typically, direct injection of the ethanol into the gas stream is preferred where the temperature of the gas stream is relatively high, i.e., well above room temperature.
To provide the non-thermal plasma environment to which the ethanol-bearing gas stream travels, the electrodes and dielectrics of the plasma chamber are spaced apart from each other and arranged in a predefined pattern defining the reaction zone through which the gas stream travels. The dielectrics are configured relative to the electrodes to isolate them from the gas stream. A voltage supply is electrically configured to apply a predefined voltage across the electrodes to create microdischarges in the gas stream.
The non-thermal plasma treatment may be used for a variety of gas streams. A typical gas stream contains approximately nitrogen, oxygen, water vapor and nitric oxide. The primary function of the treatment is to convert the nitric oxide into nitrogen dioxide. As to be described below in further detail, the introduction of ethanol significantly enhances the efficiency of the conversion in terms of the electrical power required to drive the conversion. Moreover, upon completion of the conversion of nitric oxides to nitrogen dioxide in the plasma reactor, the nitrogen dioxide and the ethanol are readily scrubbed from the existing gas stream.
These, as well as other features of the invention, will become apparent from the detailed description which follows, considered together with the appended drawings.